The n-back task is a continuous performance task that is commonly used as an assessment in cognitive neuroscience to measure a part of working memory and that has been suggested as a method for increasing fluid intelligence. The n-back was introduced by Wayne Kirchner in 1958.

The task
The subject is presented with a sequence of stimuli, and the task consists of indicating when the current stimulus matches the one from n steps earlier in the sequence. The load factor n can be adjusted to make the task more or less difficult.
To clarify, the visual n-back test is similar to the classic memory game of "Concentration". However, instead of different items that are in a fixed location on the game board, there is only one item, that appears in different positions on the game board during each turn. 1-N, means that you have to remember the position of the item, ONE turn back. 2-N, means that you have to remember the position of the item TWO turns back, and so on.